1. Field of the Invention
The present invention relates generally to a variable venturi carburetor for an engine in which the cross-sectional area of venturi portion automatically changes according to the amount of intake air to keep the vacuum generated at the venturi portion at a constant level, regardless of the amount of intake air, the carburetor of this type being called a constant vacuum carburetor. Further, in the carburetor of this type, the metering jet portion of fuel also automatically changes according to the amount of intake air to supply the mixture of a predetermined air-to-fuel ratio.
2. Description of the Prior Art
Variable venturi carburetors or constant vacuum carburetors are well known. The variable venturi carburetor is usually attached to an intake passage on the upstream side from a throttle valve. The venturi portion thereof is formed between a fixed venturi portion and a movable venturi portion. The fixed venturi portion includes a nozzle body having a nozzle portion at one end thereof, the nozzle body being connected to a float chamber to supply fuel from the float chamber to the intake passage. The movable venturi portion includes a suction cylinder, a suction piston the inner space of which is partitioned into an atmospheric pressure chamber and a vacuum chamber, and a suction spring.
The suction piston serving as the movable venturi portion moves toward or away from the fixed venturi portion, in dependence upon the force balance determined by pressure difference between the atmospheric pressure chamber and vacuum chamber, the urging force of the suction spring, and the weight of the suction piston, so that the cross-sectional area of the venturi portion changes according to the amount of intake air to keep vacuum at a constant level at the venturi portion. Further, at the center of the lower end surface of the suction piston, a tapered jet needle is fixed so as to pass through a central hole formed in the needle body. Therefore, when the suction piston moves toward or away from the fixed venturi portion, the metering jet portion formed between the jet needle and the nozzle portion of the nozzle body varies to keep the mixture obtained at the venturi portion at a predetermined air-to-fuel ratio.
However, while the engine is being idled, since the amount of intake air is very small, the vacuum generated at the venturi is too low to sufficiently absorb fuel through the metering jet portion, even if the movable venturi portion moves at its lowermost position. To overcome this problem, the prior-art variable venturi carburetor is usually combined with an additional fixed venturi carburetor including a main nozzle, a low-speed jet, a by-pass pilot fuel supply passage, and a low-speed air passage. Accordingly, there exist problems in that the structure is complicated and further it is rather difficult to obtain a stable mixture of a predetermined air-to-fuel ratio while the engine is being idled or to smoothly increase the amount of fuel according to the increase in the amount of intake air when engine load increases gradually from engine idling condition.
A more detailed description of the prior-art variable venturi carburetor will be made with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT.